Ballast for operating at least one low-pressure discharge lamp

Abstract

The invention relates to a ballast for operating at least one low-pressure discharge lamp, preferably a three- or four-lamp ballast. In order for all parallel- or series-connected low-pressure discharge lamps (L 40 -L 43 ) to shine equally bright even in the case of strong dimming, the ballast according to the invention has a transformer (Tr 40 ) for balancing the currents in the lamp branch circuits and a transformer (Tr 41 ) for compensating the losses owing to parasitic capacitances.

What is claimed is:  
     
       1. A ballast for operating a plurality of low-pressure discharge lamps, the ballast having three parallel-connected branch circuits to which electric terminals for at least one low-pressure discharge lamp are connected, wherein 
       the ballast has a first transformer for balancing the lamp currents,  
       the first transformer has a first and a second winding, the first winding having twice as many turns as the second winding,  
       the ballast has a second transformer for balancing the lamp currents, the second transformer having a first and a second winding, and the first winding having twice as many turns as the second winding,  
       the windings of the transformers are arranged in such a way that when the low-pressure discharge lamps are connected the lamp currents flow firstly through the windings of one of the two transformers, then through the low-pressure discharge lamps and only then through the windings of the other transformer,  
       the first winding of the first transformer is arranged in such a way that when the low-pressure discharge lamps are connected it is flowed through by the lamp current of the low-pressure discharge lamp or low-pressure discharge lamps connected to the first branch circuit,  
       the second winding of the first transformer is arranged in such a way that, with the low-pressure discharge lamps connected, it is flowed through by the lamp currents of the low-pressure discharge lamps connected to the second and third branch circuit, the two windings of the first transformer (Tr 10 ) being wound in opposite senses,  
       the first winding of the second transformer is arranged in such a way that, with the low-pressure discharge lamps connected, it is flowed through by the lamp current of the low-pressure discharge lamp or low-pressure discharge lamps connected to the third branch circuit, and  
       the second winding of the second transformer (Tr 11 ) is arranged in such a way that, with the low-pressure discharge lamps connected, it is flowed through by the lamp currents of the low-pressure discharge lamps connected to the first and second branch circuit, the two windings of the second transformer being wound in opposite senses.  
     
     
       2. The ballast as claimed in  claim 1 , wherein the ballast is designed as an inverter with a downstream load circuit, the electric terminals for the low-pressure discharge lamps or the branch circuits with the electric terminals for at least one low-pressure discharge lamp being arranged in the load circuit. 
     
     
       3. The ballast as claimed in  claim 1 , wherein 
       the inverter is designed as a half-bridge inverter having two switching transistors and one half-bridge capacitor,  
       the load circuit is designed as a series resonant circuit having a resonance inductor and a resonance capacitor,  
       in each case one terminal of the resonance capacitor and of the half-bridge capacitor is at frame potential,  
       the second terminal of the resonance capacitor is connected via the resonance inductor to the center tap of the half-bridge inverter, and  
       the branch circuits or the series connection or the electric terminals for the at least one low-pressure discharge lamp are switched between the second terminal of the resonance capacitor and the second terminal of the half-bridge capacitor.  
     
     
       4. A ballast for operating a plurality of low-pressure discharge lamps, the ballast having three parallel-connected branch circuits to which electric terminals for at least one low-pressure discharge lamp are connected, wherein 
       the ballast has a transformer for balancing the lamp currents,  
       the transformer has three identically constructed and isotropically arranged windings that are arranged in each case in one of the branch circuits such that they are flowed through by the lamp current in the same sense, and  
       at least one of the three parallel-connected branch circuits has electric terminals for a series circuit of a plurality of low-pressure discharge lamps, and the ballast has a second transformer with two windings with the same number of turns per unit length for the purpose of balancing the lamp currents, the first winding of the second transformer being arranged upstream of the parallel connection of the three branch circuits, and the second winding being arranged downstream of the parallel connection of the three branch circuits, such that, with the low-pressure discharge lamps connected, the two windings of the second transformer are flowed through by the total current of the branch circuits in the opposite sense.  
     
     
       5. The ballast as claimed in  claim 4 , wherein the ballast is designed as an inverter with a downstream load circuit, the electric terminals for the low-pressure discharge lamps or the branch circuits with the electric terminals for at least one low-pressure discharge lamp being arranged in the load circuit. 
     
     
       6. A ballast for operating a plurality of low-pressure discharge lamps, 
       the ballast having a transformer for balancing the lamp currents,  
       the ballast having two parallel-connected branch circuits to which electric terminals for at least one low-pressure discharge lamp are connected in each case,  
       the transformer having a first winding that is connected to the first branch circuit, and having a second winding with the same number of turns that is connected to the second branch circuit with the opposite winding sense, wherein  
       the ballast has a second transformer for balancing the lamp currents,  
       the second transformer has a first and a second winding with the same number of turns per unit length, the first winding being arranged upstream of the parallel connection of the branch circuits, and the second winding being arranged downstream of the parallel connection of the branch circuits, and  
       the first and the second windings of the second transformer are arranged in such a way that, with the low-pressure discharge lamps connected, they are flowed through in the opposite sense by the total current of the parallel connection of the branch circuits.  
     
     
       7. The ballast as claimed in  claim 6 , wherein the ballast is designed as an inverter with a downstream load circuit, the electric terminals for the low-pressure discharge lamps or the branch circuits with the electric terminals for at least one low-pressure discharge lamp being arranged in the load circuit. 
     
     
       8. The ballast as claimed in  claim 6 , wherein the first and the second branch circuits in each case have electric terminals for two low-pressure discharge lamps to be connected in series. 
     
     
       9. The ballast as claimed in  claim 8 , wherein the ballast is designed as an inverter with a downstream load circuit, the electric terminals for the low-pressure discharge lamps or the branch circuits with the electric terminals for at least one low-pressure discharge lamp being arranged in the load circuit. 
     
     
       10. The ballast as claimed in  claim 4 , wherein the first branch circuit has electric terminals for the series connection of two low-pressure discharge lamps, and the second branch circuit has electric terminals for one low-pressure discharge lamp. 
     
     
       11. The ballast as claimed in  claim 10 , wherein the ballast is designed as an inverter with a downstream load circuit, the electric terminals for at least one low-pressure discharge lamps or the branch circuits with the electric terminals for at least one low-pressure discharge lamp being arranged in the load circuit.